It is common to perform surveys by making use of one or more mobile or rover units that collect survey data at selected points. These measurements can be made using conventional (optical) surveying equipment or by making use of GPS or other satellite based augmentation system (SBAS) receivers.
As is discussed in greater detail below, in the case of SBAS, improved accuracy is achieved by making use of the relative timing of signals transmitted from a number of satellites visible to the receiver and processing the results using a technique referred to as Real Time Kinematics (RTK) to obtain highly accurate position fixes.
The Global Positioning System (GPS) is a system of satellite signal transmitters that transmit information from which an observer's present location and/or the time of observation can be determined. GPS is however not the only satellite-based navigation system. Another commonly used SBAS system is the Global Orbiting Navigational System (GLONASS), which can operate as an alternative or supplemental system to GPS.
For ease of terminology the present application will refer to a Satellite Based Augmentation System or SBAS, to refer to a GPS or GLONASS, and to any other compatible satellite-based system that provides information by which an observer's position and the time of observation can be determined.
In particular, a Satellite Based Augmentation System (SBAS) uses the transmission of coded radio signals from a family of earth-orbiting satellites (24 satellites for GPS) to calculate a receiver's position. An SBAS antenna receives SBAS signals from a plurality (preferably four or more) SBAS satellites and passes these signals to an SBAS signal receiver/processor, which identifies the SBAS satellite source for each SBAS signal, determines the time at which each identified SBAS signal arrives at the antenna, and determines the present location of the SBAS satellites, from which the receiver's location is calculated.
Differential Global Positioning System (DGPS), in turn, is a technique that significantly improves both the accuracy and the integrity of the Global Positioning System (GPS). The most common version of DGPS requires high-quality GPS “reference receivers” at known, surveyed locations. The reference station estimates the slowly varying error components of each satellite range measurement and forms a correction for each GPS satellite in view. This correction is broadcast to all DGPS users on a convenient communication link. Typical ranges for a local area differential GPS (LADGPS) station are up to 150 km and expected accuracies with DGPS are within the range from 1 to 5 meters.
Most DGPS systems use a single reference station to develop a scalar correction to the code-phase measurement. However, in what is known as wide area DGPS, or WADGPS a network of reference stations can be used instead to form a vector correction for each satellite. This vector consists of individual corrections for the satellite clock, three components of satellite positioning error (or ephemeris), and parameters of an ionospheric delay model. The validity of this correction still decreases with increased latency or age of the correction. However, compared to a scalar correction, a vector correction is valid over much greater geographical areas.
Users with very stringent accuracy requirements may further improve accuracy by making use of a technique called carrier-phase DGPS or CDPGS. These users measure the phase of the GPS carrier relative to the carrier phase at a reference site, thus achieving range measurement precision that is a few percent of the carrier wavelength, typically about one centimeter. These GPS phase comparisons are used in survey applications, where the antennas are separated by tens of kilometers. If the antennas are moving, the position fix is said to be kinematic, and is also referred to as Real Time Kinematic or RTK. Typically the position is transmitted using the National Marine Electronics Association (NMEA) protocol, wherein the kinematic information is transmitted in a GGA record in the NMEA protocol frame, for instance every 10 seconds.
Thus currently in a survey operation making use of RTK, as a rover traverses the terrain, its position, bearing, attitude and other attributes about the rover movement can be constantly monitored, and the coordinates transmitted to a central location, typically according to a standard data format protocol as defined by the NMEA (National Marine Electronics Association).
Rovers are typically connected to a server by means of a bidirectional communications link in order to allow rover units to transmit their current positions as NMEA messages to the server. GPS correction data is in turn transmitted to the various rovers and the corrected positional information sent back to the server. This typically takes place on the same line that a rover receives the reference correction stream on and is usually implemented by a radio link. In a Virtual Reference Station (VRS) environment the NMEA communications that transmit the positional information of the rovers take place between the rover units and a VRS server.
In the course of a survey operation, a rover makes survey measurements at selected locations as part of a project and stores these in a local memory, usually on a memory disk. This information is subsequently made available as a job file once the rover returns to a central station with the disk.